NASA’s Planetary Protection policy calls for the imposition of controls on contamination for certain combinations of mission type and target body. There are five categories for target body/mission type combinations. The assignment of categories for specific missions is made by the NASA Planetary Protection Officer based on multidisciplinary scientific advice. The five categories are:

Category I includes any mission to a target body, which is not of direct interest for understanding the process of chemical evolution or the origin of life. No protection of such bodies is warranted and no planetary protection requirements are imposed.

Category II includes all types of missions to those target bodies where there is significant interest relative to the process of chemical evolution and the origin of life, but where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration. The requirements are only for simple documentation. This documentation includes a short planetary protection plan is required for these missions, primarily to outline intended or potential impact targets; brief pre-launch and post-launch analyses detailing impact strategies; and a post-encounter and end-of-mission report providing the location of inadvertent impact, if such an event occurs.

Category III includes certain types of missions (typically a flyby or orbiter) to a target body of chemical evolution or origin-of-life interest, or for which scientific opinion holds that the mission would present a significant chance of contamination which could jeopardize future biological exploration. Requirements consist of documentation (more involved than that for Category II) and some implementing procedures, including trajectory biasing, the use of clean rooms (Class 100,000 or better) during spacecraft assembly and testing, and possibly bioburden reduction. Although no impact is generally intended for Category III missions, an inventory of bulk constituent organics is required if the probability of inadvertent impact is significant.

Category IV includes certain types of missions (typically an entry probe, lander or rover) to a target body of chemical evolution or origin-of-life interest, or for which scientific opinion holds that the mission would present a significant chance of contamination which could jeopardize future biological exploration. Requirements include rather detailed documentation (more involved than that for Category III), bioassays to enumerate the burden, a probability of contamination analysis, an inventory of the bulk constituent organics, and an increased number of implementing procedures. The latter may include trajectory biasing, the use of clean rooms (Class 100,000 or better) during spacecraft assembly and testing, bioload reduction, possible partial sterilization of the hardware having direct contact with the taget body, and a bioshield for that hardware, and, in rare cases, a complete sterilization of the entire spacecraft. Subdivisions of Category IV (designated IVa, IVb, or IVc) address lander and rover missions to Mars (with or without life detection experiments), and missions landing or accessing regions on Mars which are of particularly high biological interest.

Category V pertains to all missions for which the spacecraft, or a spacecraft component, returns to Earth. The concern for these missions is the protection of the Earth from back contamination resulting from the return of extraterrestrial samples (usually soil and rocks). A subcategory called "Unrestricted Earth Return" is defined for solar system bodies deemed by scientific opinion to have no indigenous life forms. Missions in this subcategory have requirements on the outbound (Earth to target body) phase only, corresponding to the category of that phase (typically Category I or II).

For all other Category V missions, in a subcategory defined as "Restricted Earth Return", the highest degree of concern is expressed by requiring the absolute prohibition of destructive impact upon return, the need for containment throughout the return phase of all returning hardware which directly contacted the target body or unsterilized material from the body, and the need for containment of any unsterilized samples collected and returned to Earth. Post-mission, there is a need to conduct timely analyses of the returned unsterilized samples, under strict containment, and using the most sensitive techniques. If any sign of the existence of a non-terrestrial replicating organism is found, the returned sample must remain contained unless treated by an effective sterilization procedure. Category V concerns are reflected in requirements that encompass those of Category IV plus a continuous monitoring of mission activities, studies, and research in sterilization procedures and containment techniques.